Geographic area is an interesting term here. If you take "iPhone owners" as a geographic area, then Apple has a 100% monopoly on that market. Geographic area == captive audience, and since iPhone users are a captive audience of the app store.

I think there's a reasonable argument that users buy iPhones with the expectation of a market in the app store (I can get or buy apps from any company that cares to develop one), so when Apple charges a 30% fee (far above normal payment processing fees) on every transaction made through the app they are abusing a monopoly position.